name used to reference a collection of descriptors; used by the client to determine which server should receive data requests

Position

absolute or time-relative position of a file in a dataset; position numbers greater than zero represent an absolute position in the dataset; position numbers less than or equal to zero represent a relative position, 0 is the most recent and -1 is the next most recent; for example, if a dataset has four images with times 13 UTC, 14 UTC, 15 UTC and 12 UTC, they have the positions -2, -1, 0 and -3